A car odometer on correctly sized, properly inflated tires is usually within 1 to 2% of the true distance, and the current UN vehicle rule allows up to ±4%. Tire wear makes it over-count by up to about 2% as the tread goes, and taller-than-stock tires make it under-count by more. To find out how far off yours is, drive 10 to 20 miles of open highway and compare the trip meter with the mile markers or a GPS app.
Here’s what the rules allow, what moves an odometer and which way, and a test you can run on your next highway trip.
How much odometer error is allowed? #
The current (02 series) version of UN Regulation No. 39, used for vehicle approval in the EU, the UK and many other countries, says the total distance shown “shall not deviate by more than ±4.0 per cent from the true distance travelled”. Unlike the speedometer rule, that tolerance runs both ways.
Speedometers are treated differently. They may never show less than your true speed and may read up to 10% plus 4 km/h above it, so carmakers set them a few percent high. Some cars carry that margin through to the odometer and some apply it only to the speed display, so a car can show 63 mph at a true 60 and still log distance within a percent. Measuring is the only way to know which kind you have.
In the US, federal odometer law targets fraud, such as rolling back mileage or misstating it when a car is sold, rather than setting an accuracy figure. US cars are generally built to the same global designs, and the same tire effects apply.
How does an odometer count distance? #
Your odometer and speedometer read the same signal: a sensor counting rotations of the wheels or the transmission output. The car multiplies those rotations by the tire circumference programmed at the factory. If the real tire is 2% smaller than the programmed one, every mile you drive gets logged as about 1.02 miles, at any speed. The error is systematic, which makes it easy to measure and, after a tire size change, possible to correct.
What makes an odometer drift? #
| Cause | Direction | Rough size |
|---|---|---|
| Worn tread (new to worn out) | Over-counts | About 1.5 to 2% on a passenger tire |
| Low tire pressure or a heavy load | Over-counts slightly | Small |
| Taller tires than stock | Under-counts | About 3.5% per extra inch on a 28-inch tire |
| Shorter tires than stock | Over-counts | Same scale, opposite way |
| Wheelspin on snow, mud or gravel | Over-counts | Varies |
| Wrong programming after a cluster or tire change | Either | Anything |
The tread figure is simple geometry. A new passenger tire has roughly 10/32 inch of tread and is worn out at 2/32. Losing 8/32 inch on each side takes half an inch off the diameter, which on a 28.5-inch tire is 1.75%. So a car on worn tires logs about 1.75 more miles per hundred than it did on new ones, and the gap snaps back when you fit a fresh set.
Taller aftermarket tires work the other way, and by more. If you’ve changed wheel or tire size, speedometer error after new tires shows how to calculate the exact difference from the sidewall.
How to test your odometer #
Use a distance you trust more than the car, and do the note-taking at a stop or have a passenger do it.
Method 1: highway markers #
- Pick a measured stretch of at least 10 miles or 15 km. Mile markers in the US and kilometre posts in many other countries are placed along the road, and a longer run shrinks the effect of exactly where you pass each one.
- Reset the trip meter as you pass the first marker. If your car needs a button held down, reset it before you start and note the reading instead.
- Check the trip reading at your next safe stop after the last marker.
- Work out the error: (trip reading − true distance) ÷ true distance × 100.
- Repeat in the other direction and average the two.
Method 2: a GPS app #
- Choose at least 20 miles (30 km) of highway with open sky, few tunnels and little stop-and-go.
- Before you leave, mount the phone where it can see the sky, plug it in, open Speedometer GPS and wait for the GPS pill to show GOOD or EXCELLENT. Reset any old trip.
- At the start, zero the car’s trip meter and press Start in the app at the same spot, while parked or with a passenger doing it.
- Drive normally. Keep the app on screen, since it only tracks while it’s open and does no background tracking.
- At the end, pull over and read both before any slow crawling through town.
- Work out the error: (trip meter − GPS) ÷ GPS × 100.
- Press Stop, then Save to keep the trip. Save doesn’t clear the readout, so press Reset before your next test.
Worked example: the trip meter shows 51.0 miles and GPS shows 50.0. That’s +2.0%. Over 12,000 miles a year, your odometer logs about 240 miles you didn’t drive, which matters on a lease with a mileage cap or when you set service intervals by the odometer.
The highway matters because of how the app filters. Speedometer GPS ignores steps under 5 m between one-second fixes so a parked phone doesn’t collect phantom distance, which means it under-counts badly below about 18 km/h (11 mph). It also leaves out any gap over 200 m, so a long tunnel’s length goes missing. Neither happens on an open highway. The other reasons GPS and the odometer part ways are in why GPS trip distance doesn’t match your odometer.
What does your result mean? #
| Odometer error | What it usually means |
|---|---|
| Within ±2% | Normal. Tread wear alone moves it this much |
| +2 to +4% | Worn or smaller tires, low pressure, or both |
| −2% or lower | Taller-than-stock tires or wrong programming |
| Beyond ±4% | Outside the UN tolerance; check tire size, then have the programming checked |
Run the test more than once. If the percentage is similar each time, it’s the car. If it jumps around, look at the routes and the GPS signal.
How do you fix an inaccurate odometer? #
Start with the cheap causes.
- Check tire pressure against the placard in the driver’s door frame, with the tires cold.
- Check the tire size on the sidewall against the size on that placard. If they differ, that’s almost certainly your error.
- Recalibrate after a size change. Dealers and specialist shops can reprogram many modern vehicles for a new tire size, and trucks on oversized tires can often be corrected with an aftermarket programmer. Some older vehicles need a different speedometer drive gear. The same fix corrects the speedometer, and how to check and calibrate a speedometer with GPS covers the options.
- Look for a fault if the error is large or erratic. A failing wheel-speed sensor often brings ABS or traction-control warning lights with it.
Never try to change the total odometer reading itself. Altering or rolling back an odometer is illegal in the US and many other countries, whatever the reason. The trip meter is yours to reset whenever you like.
A GPS app has nothing to calibrate here, because GPS distance doesn’t depend on your tires. Speedometer GPS has no tire-size or offset setting for that reason.
Which reading should you use for mileage records? #
- Service intervals, warranty and lease mileage: the odometer. It’s the official record, whatever its error.
- Business or tax mileage: consistency matters more than the last percent. Many employers and tax authorities expect odometer readings at the start and end of each trip, and a GPS log with dates and routes backs those up. Our guide to a GPS mileage log for taxes covers what to record.
- Comparing over weeks: Speedometer GPS keeps up to 100 saved trips and adds them into a lifetime odometer on the Trips screen. That total is the sum of your stored trips, so it drops if you delete old ones.
Frequently asked questions #
How accurate is a car odometer? #
Cars approved under UN Regulation No. 39 must show total distance within ±4% of the true figure. A car on the correct tires at the right pressure is usually much closer, often within 1 to 2%. Tire wear and size changes are what push it further.
Do worn tires affect odometer readings? #
Yes. Worn tires have a smaller diameter, so they turn more times per mile and the odometer over-counts. Going from new to worn-out tread typically adds about 1.5 to 2%, and new tires bring the reading back down.
Can an odometer be off by 5 percent? #
It can, usually after a change to larger or smaller tires or a programming mistake. Normal tread wear alone rarely gets you past 2%. If yours is 5% off on correctly sized tires, have a dealer check it.
Do I need to calibrate a GPS odometer app? #
No. GPS measures distance from satellite positions, not wheel rotations, so there’s nothing about your car for it to learn. Keep the phone where it has a clear view of the sky, give it a good fix before you start, and test on open highway.
Can I use a GPS app to log business mileage? #
A GPS trip log can support your records, but tax authorities set their own rules for what a mileage log must contain, such as dates, destinations and business purpose. Note your odometer at the start and end of each trip as well, and check your tax authority’s requirements.